1. Field of the Invention
This invention relates to methods for testing digital systems composed of multiple modules, e.g. printed circuit boards, each containing a number of integrated circuit chips, and more specifically to systems where some or all integrated circuit chips have leads used only during testing.
2. Discussion of the Related Art
One recurring challenge in the design and manufacture of electronic systems is control and reduction of the size of the physical interfaces between modularized elements. Such elements include, for example, integrated circuits and circuit boards. The physical interface of a modularized element is usually presented in the form of connector hardware. In integrated circuits the physical interface is presented in the form of leads; in modules, in the form of pins or pads.
It is common for test purposes to add special leads to integrated circuit chips. A widely-used example of such a test lead is an INHIBIT input lead. Whenever such a lead is driven to logic 1, circuitry inside the chip sets all the output and input/output leads of the chip to a high-impedance state. A chip's INHIBIT lead would typically be driven to logic 1 by module-level testers in order to isolate different chips within one module. The use of a lead similar to an INHIBIT lead is disclosed, for example, in U.S. Pat. No. 3,789,205 issued to R. L. James. The James patent teaches the testing of MOSFET chips which use wired-OR rather than high impedance outputs. See also U.S. Pat. No. 5,012,185 of K. Ohfuji where each chip is provided with two test input leads, allowing outputs to be switched between normal output, logic 0, logic 1 and high impedance states during test.
When a digital system containing chips with INHIBIT leads is finally shipped to customers, signal lines connected to INHIBIT leads must be permanently tied to logic 0 by wiring external to the module. Such wiring may be expensive, particularly if there are many chips-on the module, each having its own INHIBIT lead and corresponding signal line. A need is thus felt in the art for a method for using INHIBIT leads and similar test leads for module testing without having to wire all INHIBIT lines external to the module.